Stable oscillator



March 1, 1960 R. H. DlcKE 2,927,278

STABLE oscILLAToR Filed Deo. 30, 1958 2 Sheets-Sheet 2 INV EN TOR.Easier/ Ufa/YE BY/Q-rmr,

if fargwi/ STABLE OSCELATOR Robert H. Dicke, Princeton, NJ.

Application December 30, 1958, Serial No. 783,860

9 Claims. (Cl. 331-3) The present invention relates to a stableoscillator and particularly to an improved, self-oscillating atomicclock.

In the beam type molecular oscillator and in all other atomic ormolecular resonance systems of the cavity type, the resonance frequencyand line shape are affected by the cavity tuning. 'This is simply anexample of the familiar tuning interaction of two coupled, tunedcircuits. 1f, for example, the cavity dimensions should change, theoutput frequency of the atomic clock would be effected.

An object of the present invention is to provide an improved atomic ormolecular oscillator in which the cavity is always maintained tuned tothe resonance frequency of the resonant substance within the cavity.

Another object of the invention is to provide a system for maintainingthe cavity resonator on frequency in such a way that the atomic ormolecular resonance does not aect the infomation on cavity tuning andthe cavity tuning signal does not mask that of the atomic or molecularresonance.

According to the invention, the resonant frequency of the cavity andthat of the resonant substance within the cavity are compared and, whenthey are different, an error signal is produced. The error signal isapplied to the cavity resonator tuning means in a sense to maintain thecavity tuned to the resonant frequency.

An ingenious arrangement is employed for separating the information oncavity tuning from that due to the resonant substance. The cavity andresonant substance are shock excited as, for example, by suddenly andmomentarily completing a regenerative feedback loop extending frorn theoutput to the input circuit of the cavity resonator. When the cavityresonator is shock excited, it rings at the frequency to which it istuned. This ringing is the dominant signal immediately after theexcitation but it dies out rapidly (exponentially). A time gate is usedto pass the cavity resonator ringing signal (or one derived from it) toa frequency or phase comparison device. rThe resonant substance signal(or one derived from it) is passed at a later time through a frequencygate--a narrow band amplifier, to the comparison device. The narrow bandamplifier continues to ring after the excitation has been removed for atime sufhciently long for portions of the two signals (cavity ringingand amplifier ringing) to be time coincident. If, during this period,the two signals differ in frequency, an error signal is deeloped. Theerror signal may be converted to a direct current and used to controlthe cavity tuning.

The invention will be described in greater detail by reference to thefollowing description taken in connection with the following drawing inwhich:

Fig. l is a block circuit diagram of a preferred form of the invention;and

Fig. 2 is a drawing of waveforms present at various places in thecircuit of Fig. l.

Block in Fig. l represents a cavity resonator containing an excitedresonant substance. In brief, block 10 may include a vessel filled withan alkali earth metal vapor such as sodiurn, potassium, rubidium, or thelike mixed L fte-arcs Patent with a buffer gas such as helium, argon,neon, hydrogen or the like. The alkali earth metal vapor may be placedthrough optical pumping in its excited condition, that is, in acondition such that an applied microwave at the resonant frequency ofthe vapor can cause it to oscillate. An arrangement of this general typeis shown in Figs. 1 and 2 of Patent No. 2,836,722, issued May 27, 1958,to the present applicant and Thomas R. Carver.

In other forms of the invention, block 1t? may be an arrangement such asshown in Fig. 3 of the above patent and explained in detail therein. Inbrief, this comprises a two-chambered diffusion arrangement whichincludes two cavity resonators having a common apertured Wall. The gasmolecules diffuse between the two resonators. 'ihe gas may be ammonia,ethyl chlorate, or one of the other gases listed in the patent. Whenproperly excited, the gas can be made to oscillate. Y

ln another form of the invention, the gas in the cavity resonator may beplaced in its excited condition by a beam type device. In brief, a beamof the gas is passed through a high intensity electric eld and duringits passage hr-ough the field molecules of the gas in lower energystates are attracted toward the field producing means and those in thehigher energy states are focused and directed into the cavity resonator.Accordingly, the molecules in the cavity resonator are characterized bya negative Boltzmann factor temperature, that is, they are in conditionto oscillate.

For the purposes of the present discussion, it will be assumed that theexcited resonant substance in the cavity resonator is rubidium vapor.This vapor has been found to be especially suitable for an atomic clock.lts resonant frequency of interest here is 6834 megacyclesv- MW One mayassume that noise or some other transient effect has caused oscillationsat 6834 megacycles to be produced by the rubidium gas in the cavityresonator. These are applied via output lead l2 to a mixer 14.Oscillations generated in local oscillator stage 16 are applied to amixer via arms 18 and 2t) of the magic T hybridjunction 22. The localoscillator le may be a klystron, magnetron, or the like and itsfrequency shouldvbe relatively close to that of the resonant frequencyof circuit 10. To permit use of standard components in the `stagesfollowing mixer 14, the local oscillator frequency may be 6864megacycles, a frequency differing from the resonant frequency by 30megacycles.

Local oscillations are also applied via arms 1S and 24 of the magic T toa modulator stage 26. The latter may be a crystal rectifier, forexample.

The fourth arm of the magic T is preferably matched to preventreflections back to the magic T junction. This is illustratedschematically by symbol 27.

Returning to the right of the figure, the output of mixer 14, whichconsists of a 30 megacycle intermediate frequency signal, is applied towide band amplifier 218. Il`his amplifier may have a bandwidth ofseveral megacycles or so. The amplified intermediate frequency signal isapplied through gate A to the modulator 26. It may be assumed for thepresent that gate A is open (the feedback loop is closed). The signalproduced by modulator 26 is equal to fo-530 megacycles 3G megacycles=f,the resonant frequency of the rubidium gas. The delays and amplificationin the feedback loop are such that there is regeneration and therubidium continuously oscillates at the resonant frequency.

A portion of the 30 megacy-cles signal at lead St) is applied via lead32 to the resonant substance signal channel 34 and the cavity ringingsignal channel 36. The former channel consists of a gate B and a narrowband amplifier 38. The amplifier has a bandwidth of the order of athousand to ten thousand cycles, for example. The cavity ringing channel36 includes a gate Cl The -cavity dimensions.

Ysignal decays `exponentially). Y passed by 4gate B is'predomin'a'ntlythe one due to the band'aniplifier 3S. Y q n Y The interval vduringwhich the gas signal vand the cavity.

Viilt'erilZ-to a D.C. tuningsignal and the latter isi-applied tothecavity resonator Via lead 44.

Th'etunmg means inthe cavity resonator may be any one of---several wellknown types and accordingly is illustrated by single block'l. Forexample, the signal may be'appliedto ya heating element which thermallycontrols the cavity dimensions, VAlternatively, the error signal'can beapplied Yto a servo motor which controls theY Vcavity dimensions Vor theinsertion of a tuning element into-the cavity resonator. Alternatively,the tuning signal vcan be applied `to a lreactive crystal in the cavitywhich lcon'ti'ols'the cavity tuning.

4 nal are present.y The balanced phase detector 40 compares signals dand f during the interval t1 to t2. When the signals are of the samefrequency, no output is produced -by the phaser detector, however, whenthey are different, an error signal is produced. This error signal ischanged by low pass lfilter 42 to a D.C. tuning signal which, as alreadymentioned; is fed back to the cavity resonator tuning element. v Y

It is desirable to maintain the local oscillator `frequency stable. Onemethod of improving the stability is to detect the 30 megacycle signalat lead 46 and to emt ploy the detected signal in a standard frequencycontrol YPulser 46' produces the gate pulses which open and clos'e'gatesA,"B,` and C. This, too, is a standard circuit and -nay consist ofsingly stable multivibrators, forex- Vample, eachV triggered' bythelagging edge of the pulseY of therprecedingV stage. Y Y l Theoperation of the system can be better understood Yby-'refe'rringy toFig. 2. Pulse'a is shortl and is'applied from pulser 46 to gate A inasense to open the gate and v thereby to close the feedback loop.V Duringthe gate interval tu to t1 the rubidium inthe cavity resonatorYosc'i'llates,` the feedback-sustaining the oscillations. Duringtheremainder of each cycle, that is, :1 to to the rubidiumcontinues to ringat its resonant frequency of 6834 f nieg'acycles." The lamplitude of theresonance `decreases` v'ery slightly during this interval as theVrelaxation time- Vis relatively long. This decrease in what Vemphasizedin Fig: .2b

During the'interval to' to t1', that'is, the resonantSubstanca-excitation interval, theV cavity resonator is alsoV excitedfand the cavity ringing builds up to a signalV of high "amplitude At Vtime t1 the cavity ringing signal amplitude has attained a very highvalue, however, thereafter it 'decays-exponentially as'shown in Fig. 2d.At

V: time t1, lthe cavity ringing signal is many times stronger Vthan thegas ringing signal, the difference in amplitudes being much greater thanthat shown inthe drawings. The

frequency .of the cavity ringing signal depends on the Forroptimumsystem performanceV thefrequency-of the cavity ringing signal should bevprecisel'ythe 4same asrth'e resonant frequency of the rubid- Thecavityringing signal is allowed to pass to the cavityY ringingsignalchannel 36 by opening'gate C.. duringthe period immediately aftergate A has been'opened. r[The Vgate pulse which opens gate C is shown inFig. `v2c.

.The duration of this 1gate puse tlg-to z2 is preferentiallysufficiently long to permit the cavity ringing signal amplitude to decayto a low value.

It should be appreciated that during pulse c, there is also present onlead32 the signaldue to therrubidium.V

However, this signal is initially of much smaller-'ampliarrangement. A4direct kcurrent is produced which is indicative of any change in Ythefrequency of oscillator i6. It may be fed back to a tuning element inthe local oscillator for stabilizing the local 'oscillator frequency.

A preferred way of stabilizing the local oscillator is with the circuitcomprising phase detector 48, reference oscillator 47 and frequencycontrol lead 49. A lchange in the tuning of the local oscillator 16results in a change inthe megacycle signal derived from the resonantsubstance. The resulting phase shift of this signalrelative to thereference oscillator 47 producesa tuning signal Yto keep the llocaloscillator correctly tuned, YAn integrator 50 Vis preferably placedbetween the phase detector and local `oscillator for smoothing theD.'C.'tuning signal.

Both-of these means of controlling the tuning of the derived-from theresonant substance at the predetermined amplitude is some- Y tuile-'thanthe signal due to the cavity ringing and Vmay be ignored foral-lpractical purposes. Y

After time r2, pulser 46 applies a gate pulse e to gate B5' This pulsehas'a duration t2 to t0'-the remainder of the period between the gatepulses applied to gate A.VK

During the interval t2 to to the cavityk ringing signal has all butdisappeared "(it 'must be remembered that this Y Accordingly, the signalrubidium. It is amplified to the proper level by narrow ringing signalare compared is t1 to t2. During this interval gate Bris closed.VHowever, the narrow band amplilierv continues Vto-ring at the 30megacycle resonant 'fre- Vquency *of the'amplifer just asv the resonant`substance continues-toY ring at its resonant frequency.Accordinglygduringthe interval t1 to z2 lboth the V30 megacyclesigfrequency of the narrow band amplifier. Hence, the ringing frequencyof this amplifier is equivalent to the frequencyfderived from theresonantrsubstance.

V If desired, theyoscillator 47 to which the signal' from the resonantsubstance isV locked may besupplied to the phase detector 49in placeofthe signal from the narrow band amplifier 38. In thiscase, thenarrow-band amplifier is not necessary and may be omitted.

What is claimed is:

1. lnapparatus in which an excited resonant substance is in a cavityresonator, an arrangement for maintaining the cavity resonator tuned toa resonant frequency of the substance comprisiilgin Combination, meansfor comparing the resonant frequency of the cavity resonator with thatYof the resonant substance and, when 11h65' are different, producing anerror signal; and means responsiveM-to said error signalpfor maintainingthe' cavity resonator tuned to said resonant 'frequencyfofsaidsubstance, Y 2.I|n.con1'bination,za `cavi-ty resonator. -tun'ed to aresonant frequency'of a given resonant substance; a given resonantsubstancein said cavity resonator; Vmeans for shock exciting-said cavityresonator -andf resonant substance, whereby the cavity resonator ringsat a frequency to which'it'ris resonant and the-.resonant substanceoscilla'tes at a frequency to which it is resonant, the ringing of thecavity resonator Ibein-g of much greater amplitude than the oscillationof the resonant substance immediately after the shock excitation;V meansfor phase comparing the signal*l outputfrom the cavityresonatorrimmediately after it isy shock 'excited withfthersignalproduced at a later time by the resonant substance; and means responsiveto a difference in frequency between the two compared lsignals fortuning said cavity resonator.

3..An Varrangement for separating information concerning the frequencyto which a cavity resonator is tuned from infomation concerning Vthefrequency of an excited resonantn substance within-,the resonatorcomprising, in

Y combination, means forl'shock eXciting the. cavity resonator, wherebythe resonator rings at the frequency to which it is tunedj'andtheisubstance emits energy at a resonance frequency which issubstantially equal to the decaying 'exponentially with time; wherebythe `amplitude ...A IN es,

of the cavity ringing is shortly substantially smaller than thereso-nant substance signal; means for sensing the cavity ringing signalduring the period it is of substantially larger amplitude than theresonant suostance signal; and means for sensing the resonant substancesignal during the time it is of substantially larger amplitude than thecavity ringing signal.

4. An arrangement for separat-ing information concerning the lfrequencyto which a cavity resonator is tuned from information concerning thefrequency of an excited resonant substance within the resonator comprising, in combination, means for shock exciting the cavity resonator,whereby the resonator rings at lthe frequency to which it is tuned andthe substance emits energy at a resonance frequency which issubstantially equal to the frequency to which the cavity resonator istuned, the initial amplitude of the cavity ringing being much largerthan the resonant substance signal and said cavity ringing decayingexponentially with time, whereby the amputude of the cavity ringing isshortly substantially smaller than the resonant substance signal; meansfor sensing the cavity ringing signal during the period it is cfsubstantially larger amplitude than ithe resonant substance signal;means for sensing the resonant substance signal during the time it is ofsubstantially larger amplitude than the cavity ringing signal; and meansfor comparing the frequency of the two signals sensed to produce anerror signal when the two signals dier in frequency.

5. An arrangement for separating information concerning ythe frequencyto which a cavity resonator is tuned from information concerning thefrequency of an excited resonant substance within the resonatorcomprising, in combination, means for shock exciting the cavityresonator, whereby the resonator rings at the frequency to which it istuned and the substance emits energy at a resonance frequency which issubstantially equal to the frequency to which the cavity resonator istuned, the initial amplitude of the cavity ringing being much largerthan the resonant substance signal and said cavity ringing decayingexponentially with time, whereby the amplitude of the cavity ringing isshortly substantially smaller than .the resonant substance signal; agate circuit; means for passing the cavity ringing signal and resonantsubstance signal through said gate circuit during the period where theformer is of much larger amplitude than the latter; a second gatecircuit; and means for passing the cavity ringing and the resonantsubstance signal through the second gate circuit at the time the cavityringing is of substantially smaller amplitude than the resonantsubstance signal.

6. In an atomic clock, a cavity resonator; a resonant substance withinthe resonator which oscillates yat the resonant frequency of the cavityresonator; a regenerative feedback loop extending from the output to`the input circuit of said cavity resonator for maintaining saidsubstance oscillating; means for eectively closing and then opening saidfeedback loop, thereby shock exciting said cavity resonator, whereby theresonator rings at the fre- 23 quency to which it is tuned and thesubstance emits energy at its oscillating frequency, the `initialamplitude of the cavity ringing being much larger than the resonantsubstance siguel said cavity ringing decaying exponentially w time,whereby the cavity ringing is shortly substantially smaller than theresonant substance signal; means for sensing the cavity ringing signalduring the period it is of substantially larger amplitude than the`resonant substance signal; means for sensing the resonant substancesignal during the time it is of substantially larger `amplitude than thecavity ringing signal; means for comparing the two signals sensed and,when they are dierent in frequency, producing an error signal; and meansIresponsive to said error signal for maintaining said cavity resonator:tuned to the frequency at which said resonant substance osclllates.

7. In the combination as set forth in claim 6, said means for sensingthe cavity ringing signal comprising a time gate circuit, and said meansfor sensing the resonant substance signal comprising a narrow bandamplifier.

8. A stable oscillator comprising, -in combination, a cavity resonator;a resonant substance within the resonator which oscillates at theresonant frequency of the cavity resonator; an oscillator operating at afrequency close to that of said resonant substance; a mixer to which theoscillator signal and resonant substance signal are applied forproducing an intermediate frequency signal; a modulator to which theintermediate frequency signal and the oscillator signal are applied forproducing a sum signal; means for applying said sum signal to saidcavity resonator so as to maintain said resonant substance oscillating;means for closing and then opening the circuit between said mxer andmodulator thereby shock exciting said cavity resonator, whereby thecavity rings at an initial amplitude much larger than that of theresonant substance signal, said cavity inging decaying exponentiallywith time, whereby the amplitude of the cav-ity ringing yis shortlysubstantially smaller than the resonant substance signal; rst and secondgate circuits connected to receive said intermediate frequency signal;means for closing said first gate circuit during the period when thecavity ringing is of substantially larger amplitude than the resonantsubstance signal; means for closing the second gate circuit when theresonant substance signal is of substantially larger amplitude than thecavity ringing; means for comparing the frequencies of the signalspassed by the two gate circuits and, when they are dilerent, deriving acavity tuning signal; and means responsive to said cavity tuning signalfor maintaining said cavity resonator tuned to the resonant frequency ofsaid resonant substance.

9. In the combination as set forth in claim 8, said second gate circuitincluding a narrow band amplifier which continues to ring at theintermediate frequency after said second gate circuit is closed.

No references cited.

